HONO formation mechanisms and impacts on ambient oxidants in coastal regions of Fujian, China

Abstract. Nitrous acid (HONO) is a vital precursor of hydroxyl radicals (OH) in the troposphere, leading to the formation of secondary air pollutants, including ozone (O3) and secondary aerosols. Previous studies have mainly focused on investigating the chemical fate of HONO in polluted urban areas of China and found a general diurnal variation featuring the minimum concentration around noon due to the fast self-photodissociation. However, this study reported a significantly higher daytime HONO concentrations based on one-month measurement during May 2024 over the coastal regions of Fujian in southeastern China. Using an updated Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model, we captured the magnitude and temporal variation of HONO concentrations observed in coastal areas, and improved the model performance on diurnal patterns of the NO2 and O3. Further process analysis revealed that two light-dependent chemical sources, i.e., the heterogeneous uptake of NO2 on the ground surface and NOx photo-oxidation, were the main contributors to HONO formation, particularly at high concentrations around noon in the presence of persistent intensive solar radiation. In addition, we assessed that shipping emissions contributed 20 % to the midday HONO production rate in coastal regions. Subsequently, model results indicated that HONO photolysis accounted for 34 % of primary OH sources during the daytime. Model sensitivity experiments demonstrated that incorporating multiple HONO sources increased the daily maximum OH and average O3 concentrations by 61 % and 44 %, respectively, in coastal regions. Overall, this study highlights the unique formation mechanisms of HONO and its significant contribution to ambient oxidants in typical coastal regions.